With the rapidly developed information technology, various types of electronic products, such as computers, have become very important and indispensable tools in people's daily life and work. Generally, to upgrade the computer operating performance, electronic components working inside the computer, such as the central processing unit, would consume higher power. As a result, higher thermal effect on the electronic components is produced, and the work curve of the computer tends to shift when the internal temperature of the computer gets higher and higher to thereby cause unstable operation of the internal electronic components. Under this condition, the computer tends to have faulty operation or even become down. In a worse situation, the internal electronic components of the computer will be burned out due to the high internal temperature of the computer. Therefore, it is not difficult to understand why the computer heat dissipation is always a problem deeply confusing the information industry. Now, the use of a cooling fan to carry away the heat produced by the electronic components inside the computer has become a prerequisite part of the computer configuration. While the fan carries away the heat from the computer for the computer to operate stably, the fan also produces noise during the operation thereof to bring another confusing problem.
The problem of fan noise has received high attention of the information and electronic industries, because noise tends to cause different degrees of discomfort and anxiety to people, make people feel tired easily to reduce work efficiency and suffer from mental and physiological detriment. Therefore, effective noise elimination is not only meaningful in terms of technical research fields, but also practical in terms of industrial production and daily life improvement.
Currently, the ways for suppressing fan noise can be divided into two types, namely, passive noise suppression and active noise control (ANC). In the active noise control, a secondary sound source is utilized to interfere and destruct the noise source and thereby achieve the purpose of noise control. The active noise control necessitates the use of a personal computer to effectuate the control and therefore requires relatively high cost. On the other hand, the passive noise suppression refers to the reduction of noise level through improvement made to fan components, such as design changes in the shape of fan frame, the shape of fan blades, the stationary blades, and the ribs. However, when it is desired to make these design changes while maintaining the heat dissipation effect, the fan would usually produce an even higher noise value. Moreover, the passive noise suppression has only limited effect in attenuating low-frequency noise.
In brief, the conventional passive noise suppression and active noise control have the following disadvantages:    1. Changing the design sizes of or the material for making different fan components according to the passive noise suppression would increase the cost of fan and has little effect in terms of low-frequency noise.    2. Mounting additional passive blades or frames for the purpose of reducing fan noise would result in early occurrence of resonant frequency produced by the fan, which will in turn cause reduced usable life of the fan and damage the electronic components inside the computer.    3. The active noise control necessitates the use of a personal computer to effectuate the control and requires increased unit cost, and is therefore not economical for use.
It is therefore desirable to overcome the problems and disadvantages in the conventional passive noise suppression and active noise control.